Time lapse allowed between work allegedly done and invoicing?

I had an accountant that did some work for me in October 2006 to April 2007. I have paid all invoices that have been presented. In May 2008 I received a letter stating that "having undertaken a detailed review of his time ledgers" there is some unbilled time relating to work done for me. I have queeried this bill and the time taken for me to receive it and my reply from the accountant was that I was lucky to have had an interest free loan from him! Please can somebody advise me of my best course of action. Thank you in anticipation.